This invention relates to an assembly of a thermographic stencil sheet of the type which includes an ink-impervious coating of a heat-flowable composition on an ink-pervious base sheet, and a receiving sheet, and to a method of making an imaged stencil sheet with the assembly by subjecting image areas of the stencil sheet to heat generated by infrared ray absorption.
A thermographic stencil sheet including an ink-pervious base sheet and an ink-impervious coating thereon of a heat-flowable composition of thermoplastic film-forming material comprising a cellulose organic ester, and plasticizing material partially but incompletely compatible with the film-forming material is disclosed in various embodiments in U.S. Pat. Nos. 3,694,244, 3,694,245, 3,704,155, 3,824,116 and 3,824,117. The stencil sheet now is in widespread commercial use. It is employed preferably in a stencil sheet assembly including a contacting receiving or absorbent sheet on one surface thereof, and a more rigid backing sheet on the opposite surface thereof and to which the receiving sheet and the stencil sheet are mounted. In use, an original, such as a typed or printed sheet, is inserted between the stencil sheet and the backing sheet, and the assembly is exposed to infrared radiation on the face side of the receiving sheet in a thermocopier such as a Weber Thermal Imager (Weber Marking Systems) or a Thermo-Fax machine (3M Company), whereby the stencil sheet is imaged. Heat is generated in the radiation absorptive graphic portions of the original during imaging, to cause the stencil sheet composition to flow in corresponding areas and thereby produce corresponding image openings in the stencil sheet. A portion of the composition rendered flowable adheres to and/or is absorbed by the receiving sheet and/or adjoining areas of the stencil sheet. The original and the receiving sheet are separated from the imaged stencil sheet, the stencil sheet and the backing sheet are placed on a mimeograph duplicating machine followed by a separation of the backing sheet, and the machine is operated to produce multiple mimeograph copies of the original.
Copies of the best quality are produced from stencils which have been imaged at their optimum imaging speeds or exposure times. Optimum imaging speed is, however, subject to variations in infrared ray absorptivity of the original variation among stencils of different composition, variation from lot to lot of stencils of a given composition, and variation with different machines and power supplies. Consequently, it is necessary or preferable to establish the optimum imaging speed under each new set of conditions. Prior to the present invention, stencils have been evaluated for degree of imaging for the most part by placing them on a mimeograph machine and running off copies. They may be evaluated by holding them up to the light after removing the original, but it then is no longer possible to rerun an under-exposed stencil, and a certain amount of proficiency is required in order to evaluate the stencil without making copies therefrom. Consequently, a need exists for a better method of proofreading a stencil for the degree of imaging, or of determining the optimum setting on an imager.